Modems are used for transferring information on communication lines or other communication media between two parties. The modem converts information from electrical signals on the communication line to data bits, and vice versa. In the past, nearly all modems used in homes and small offices operated by dial-up over telephone lines and were limited to low data rates, typically no more than 56 kbps. Recently, however, broadband modem technologies have been developed, such as Digital Subscriber Line (DSL) systems, cable modems and fixed wireless data links.
Asymmetric DSL (ADSL) service, for example, offers downstream service at rates up to 8 Mbps. ADSL is based on a discrete multi-tone (DMT) transmission system, in which data are encoded using 256 different “tones,” or “bins,” each corresponding to a frequency band 4.3125 kHz wide. The lower bins (typically bins 6–31) are assigned to upstream service, from customer premises to a central office, while the upper bins are used for the downstream. Each DMT symbol is a mapping of a fixed number of bits to sine waves of multiple frequencies. Further aspects of ADSL are defined in Recommendation G.992.1 of the International Telecommunication Union (ITU), which is incorporated herein by reference. A similar DSL system, likewise based on DMT, is described in ITU Recommendation G.992.2 (also known as G.lite). DMT transmission is also used in other broadband communication standards.
In many homes and offices, a broadband data channel is shared among multiple clients, typically personal computers (PCs). This purpose is commonly achieved by connecting the clients to the broadband modem over a local area network (LAN), such as an Ethernet LAN. The modem unit is supplied with an Ethernet output. A router is attached to the Ethernet output of the modem (either as a standalone unit or integrated into the modem box), and controls communications between the modem and the different clients. A typical router of this sort has several Ethernet ports, each connecting to a different client, along with suitable switching logic for arbitrating among the clients. Alternatively, a wireless LAN may be used, with a wireless “access point” taking the place of the router.
There are several problems with using a LAN to share a broadband channel and modem resources among clients. The router or access point that must be used adds to the cost of the system. When the customer premises do not have a LAN already in place, it is also necessary to add wiring, LAN adapters and software on all the client computers that are to use the broadband channel. In addition, conventional LAN-based solutions cannot readily accommodate soft modems running on the clients, since soft modems require an uninterrupted flow of samples and significant guaranteed bandwidth.